Two-state electronic switches are widely used in modern electronics, from basic circuits to microprocessors to electronic memories. An electronic memory comprises a large number of ordered and electrically indexed switches, the state of each switch representing a binary “1” or “0” value or, in other words, a bit of information. Many different types of electronic memories are currently used in computers, in various types of intelligent electronic devices and controllers, and in many popular commercial products, including digital cameras and video recorders. While many applications require dynamic electronic memories that can be repeatedly written and read, many other applications require only static electronic memories that can be written only once, and then read repeatedly. Example write-once, read-many-times (“WORM”) memory devices include commonly used and optically accessed write-once compact discs, various well-known, read-only memories (“ROMs”), and various types of electronically accessed, microfuse-based crosspoint memories. WORM memories may be significantly less expensive than memories that can be repeatedly read and written, and may find great utility in applications requiring large, robust and resilient memory devices, including digital photography, where inexpensive WORM memories may be used for storing digital images in a fashion analogous to storing analogue optical images on photographic film.
In order to produce inexpensive and robust WORM memories, the process by which the millions of switches that together comprise a memory are fabricated needs to be as simple as possible, using only a small number of easily automated steps to produce a functional electronic data-storage medium. Moreover, for use in consumer devices, it is important that the electronic memory be relatively robust and relatively insensitive to mechanical shock, temperature changes, slow chemical degradation, and a variety of other environmental insults to which consumer products are commonly exposed and other types of inherent physical instabilities. Simplicity of design may, by itself, greatly facilitate robustness, because every additional component or feature may represent an additional failure point. Thus, for a variety of reasons, inexpensive, stable, and easily manufactured WORM memories are of great potential utility in current and future consumer products. Designers, manufacturers, and users of electronic memories, and, in particular, designers, manufacturers, and users of consumer devices that include inexpensive WORM memories, have thus recognized the need for robust, inexpensive, and easily manufactured WORM memories.